You need to include the OWP (Form IMM5710) with the Inland application package (in the same envelope). This way, CIC will know that you have applied for SCLPC sponsorship (Inland application). Make sure to pay the full amount for the OWP, which is $255 (unless it has gone up). Check the CIC website before mailing.
If you apply online, it's possible that CIC will start to process the OWP before they have acknowledged receiving the Inland application, thus leading to a refusal and a waste of your money.
The above is based on a person that still has legal status in Canada.